Du kannst da noch nicht reingehen.

I feel like I have a pretty good grasp of the hin / her thing, but this one confuses me. “rein” seems short for “herein”, which I thought meant “Into the place where the speaker is”. So, if the speaker is saying “Da”, i.e. going “there”, somewhere the speaker is not, wouldn’t you say “hineingehen”?

Is this one of those things where in casual speech you just throw out this disinction and use “reingehen” for both? Or am I missing something here?

rein doesn’t imply anything of the above. hinein and hereingehen don’t differ in terms of what they mean, its only that hinein is used more frequently in the north whereas herein is used rather in the south. reinkommen is casual speech used everywhere.